Method for treating exhaust gas containing fluorine-containing compound

ABSTRACT

The present invention aims to provide exhaust gas treatment processes and systems capable of efficiently treating perfluoro-compounds (PFCs) for a long period. It also aims to provide exhaust gas treatment processes and systems capable of efficiently treating oxidizing gases such as F 2 , Cl 2  and Br 2 ; acidic gases such as HF, HCl, HBr, SiF 4 , SiCl 4 , SiBr 4  and COF 2 ; and CO in addition to PFCs. 
     In order to attain these objects, an exhaust gas treatment process of the present invention is a process for treating an exhaust gas containing a fluorine-containing compound with a catalyst after solids in said exhaust gas have been separated, said catalyst being a γ-alumina having a crystal structure showing diffraction lines having an intensity of 100 or more at the following five diffraction angles 2θ: 33°±1°, 37°±1°, 40°±1°, 46°±1° and 67°±1° measured by an X-ray diffractometer. An exhaust gas treatment process according to a preferred embodiment of the present invention is characterized in that at least one decomposition-assisting gas selected from H 2 , O 2  and H 2 O is further added to the exhaust gas.

TECHNICAL FIELD

The present invention relates to processes for treating exhaust gasescontaining fluorine-containing compounds, particularly to exhaust gastreatment processes and systems capable of efficiently and long treatingperfluoro-compounds discharged during the step of dry-cleaning the innerfaces or the like of semiconductor manufacturing apparatus withperfluoro-compounds such as C₂F₆, C₃F₈, CF₄, CHF₃, SF₆, NF₃, etc. or thestep of etching various films in the semiconductor industry. In morepreferred embodiments, the present invention relates to exhaust gastreatment processes and systems capable of efficiently treatingoxidizing gases such as F₂, Cl₂ and Br₂; acidic gases such as HF, HCl,HBr, SiF₄, SiCl₄, SiBr₄ and COF₂; and CO in addition to theperfluoro-compounds.

BACKGROUND ART

In the semiconductor industry, many kinds of noxious gases are usedduring semiconductor manufacturing processes, which raises concernsabout environmental pollution. Perfluoro-compounds (PFCs) contained inexhaust gases from etching processes or CVD processes are global warminggases for which a removal system should be urgently established.

PFC removal processes have been proposed such as destruction andrecovery techniques, especially destruction techniques including variouscatalytic pyrolysis methods. For example, proposed prior processesinclude an exhaust gas treatment process using an alumina-based catalystloaded with various metals; an exhaust gas treatment process usingalumina containing 0.1% by weight or less of Na as metal; an exhaust gastreatment process involving contacting exhaust gases with molecularoxygen in the presence of alumina; a process for treating exhaust gasescontaining fluorine-containing compounds using an Al-containing catalystin the presence of water vapor at a temperature of 200-800° C.; aprocess for treating exhaust gases containing fluorine-containingcompounds using various metal catalysts in the presence of molecularoxygen and water; etc.

However, these prior proposals had the problems that fluorine-containingcompounds were decomposed with still too low efficiency to obtainsufficient treatment performance or a long continuous treatment couldnot be attained with a given treatment system because of the short lifeof the alumina catalysts.

Thus, an object of the present invention is to solve these problems ofthe prior art and to provide a process and a system for treating exhaustgases containing fluorine-containing compounds with high PFCdecomposition efficiency, which allows PFCs to be effectively decomposedand eliminated for a long period.

DISCLOSURE OF THE INVENTION

As a result of careful studies to solve the above problems, we foundthat the above object can be achieved by using an alumina having aspecific crystal structure among those having various crystalstructures. Accordingly, the present invention relates to a process fortreating an exhaust gas containing a fluorine-containing compound with acatalyst, characterized in that said catalyst is a γ-alumina having acrystal structure showing diffraction lines having an intensity of 100or more at the following five diffraction angles 2θ: 33°±1°, 37°±1°,40°±1°, 46°±1° and 67°±1° measured by an X-ray diffractometer.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a schematic flow diagram of an exhaust gas treatment systemaccording to an embodiment of the present invention.

THE MOST PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is further described in detail below.

First, the γ-alumina having the crystal structure defined above used inthe present invention is explained.

Basically, activated alumina refers to an intermediate between hydrates(trihydrate: Al₂O.3H₂O, monohydrate: Al₂O₃.H₂O) and α-alumina (having adense structure), and is represented by Al₂O₃.

Activated alumina is classified into seven types (κ:kappa, θ:theta,δ:delta, γ:gamma, η:eta, χ:chi and ρ:rho) of metastable aluminas. Thesealuminas are obtained by heat treatment of hydrated alumina and contain0-0.5 moles of water per 1 mole of Al₂O₃ depending on the temperature ofthe heat treatment.

γ-alumina is one of these activated aluminas and said to be unstable andhighly active when it has a specific crystal structure (the pattern ofX-ray diffraction peaks). Various γ-aluminas having different activitiesare produced depending on the nature of the hydrate or the type of thepreparation process. We examined such γ-aluminas having various crystalstructures for their performance of decomposing fluorine-containingcompounds depending on the difference in crystal structure to find thata γ-alumina having a crystal structure showing diffraction lines havingan intensity of 100 or more at the following five diffraction angles 2θ:33°±1°, 37°±1°, 40°±1°, 46°±1° and 67°±1° measured by an X-raydiffractometer (the γ-alumina having this crystal structure ishereinafter referred to as “the present γ-alumina”) has especiallyexcellent decomposition performance, and we accomplished the presentinvention on the basis of this finding.

The γ-alumina having such a crystal structure can be obtained by, forexample, sintering alumina sol into spherical alumina hydrogel(Al(OH)_(y).nH₂O).

The Na₂O content in the present γ-alumina is preferably 0.02 wt % orless in the total amount of the γ-alumina in view of the performance ofdecomposing fluorine-containing compounds.

The γ-alumina used in the present invention can be in any shape so faras it has the crystal structure defined above, but it is preferablyspherical for ease of handling. The particle size of the γ-alumina usedin the present invention is preferably smaller to provide a largercontact area unless the resistance to gas transmission increases duringgas feeding, especially 0.8 mm-2.6 mm.

Specific examples of such γ-alumina are commercially available fromMizusawa Chemical under trade name “Neobead GB-08”, for example.

In order to perform exhaust gas treatment processes of the presentinvention, an exhaust gas containing a fluorine-containing compound canbe treated by using the present γ-alumina having the crystal structuredefined above as a catalyst. Preferably, the present γ-alumina is heatedat 600-900° C., more preferably 650-750° C.

Fluorine-containing compounds that can be treated by the presentinvention include fluorohydrocarbons such as CHF₃; andperfluoro-compounds (PFCs) such as C₂F₆, C₃F₈, SF₆, NF₃, etc.

Said exhaust gas containing a fluorine-containing compound may beexhaust gas discharged during the step of dry-cleaning the inner facesor the like of semiconductor manufacturing apparatus or the step ofetching various films in the semiconductor industry.

In a preferred embodiment of the present invention, a process capable ofnot only efficiently decomposing PFCs but also treating oxidizing gases,acidic gases and CO is provided.

Exhaust gases discharged from semiconductor manufacturing processescontain not only PFCs but also oxidizing gases such as F₂. Cl₂ and Br₂;acidic gases such as HF, SiF₄, COF₂, HCl, HBr, SiCl₄, and SiBr₄; and CO.Oxidizing gases such as F₂, Cl₂ and Br₂ had the problem that they couldnot be completely treated with water but required the use of an alkaliagent or a reducing agent when they were to be wet-treated, whichresulted in complicated control or apparatus and added costs. CO isgenerated as a by-product during decomposition of PFCs and must bedecomposed and eliminated.

In a preferred embodiment of the present invention, the exhaust gas tobe treated is combined with at least one decomposition-assisting gasselected from H₂, O₂ and H₂O and subjected to the treatment describedabove. By using such a decomposition-assisting gas, the catalyst life ofthe present γ-alumina can be further remarkably extended and the exhaustgas can be efficiently treated for a longer period. The CO generated asa by-product during the decomposition process of PFCs can also beefficiently decomposed.

When said decomposition-assisting gas is added, fluorine-containingcompounds such as PFCs, oxidizing gases and CO are decomposed intoacidic gases and CO₂ according to the following formulae.

CF₄+2H₂+O₂→CO₂+4HF

 CF₄+2H₂+O₂→CO₂+4HF+O₂

F₂+H₂→2HF

2F₂+2H₂O→4HF+O₂

2CO+O₂→2CO₂.

Namely, CF₄ is decomposed into CO₂ and HF by the reaction with H₂ and O₂or H₂O, oxidizing gases such as F₂ are decomposed into acidic gases suchas HF by the reaction with H₂ or H₂O and CO is oxidized into CO₂.

H₂, O₂ and/or H₂O used to treat PFCs here are preferably added in anamount of H₂ and/or H₂O equivalent to or more than the necessary numberof moles for converting F atoms in PFCs into HF and in an amount of O₂equivalent to or more than the necessary number of moles (minimum) forconverting C atoms into CO₂. More preferably, H₂ and/or H₂O is 6-20times the equivalent per mole of PFCs, while O₂ is equal to or more thanthe above minimum plus 1 mole. For treating oxidizing gases, H₂ ispreferably added in an amount equivalent to or more than the necessarynumber of moles for converting halogen atoms (X) in the oxidizing gasesinto acidic gases (HX).

An embodiment of a system for performing a treatment process of thepresent invention comprises a solids separator for separating solids inthe exhaust gas, a catalytic treatment apparatus packed with the presentγ-alumina having the crystal structure defined above and an acidic gastreatment apparatus, which are connected via piping.

The solids separator and the acidic gas treatment apparatus here are notspecifically limited, but can be any commonly known ones. For example,water scrubbers (water spray columns) or the like can be used as thesolids separator and the acidic gas treatment apparatus.

The catalytic treatment apparatus described above preferably has aheater for heating the γ-alumina catalyst of the present invention.Specifically, the catalytic treatment apparatus may consist of acylindrical column packed with the present γ-alumina and surrounded by aheater around the outer peripheral face and connected at the top to anexhaust gas feed pipe and at the bottom to a discharge pipe for treatedexhaust gas.

An exhaust gas treatment process according to a preferred embodiment ofthe present invention can be performed as follows, for example.

An exhaust gas is first passed through the solids separator at apreliminary stage where solids are eliminated. The γ-alumina of thepresent invention in the packed column is heated by the heater at atemperature of 600-900° C., and then the exhaust gas freed of solids iscombined with the decomposition-assisting gas and passed through thecatalytic treatment apparatus packed with the γ-alumina wherefluorine-containing compounds such as PFCs are decomposed into acidicgases and CO₂ simultaneously with oxidizing gases and CO by thecatalytic action of the γ-alumina. Thus, the decomposition-assisting gasand the exhaust gas are preferably injected as a mixed gas into acatalyst bed formed by packing the catalyst.

Only acidic gases (HX) and CO₂ exist in the exhaust gas exiting thecatalytic treatment apparatus, and the acidic gases can be eliminated inthe acidic gas treatment apparatus such as a water spray column tocomplete exhaust gas treatment.

FIG. 1 shows a schematic flow diagram of an exhaust gas treatment systemaccording to a preferred embodiment of the present invention. In FIG. 1,various references represent the following elements: 1: solids separator(water spray column); 2: packed bed of γ-alumina having a specificcrystal structure according to the present invention; 3: catalytictreatment apparatus; 4: washing water circulating pump; 5: acidic gastreatment apparatus (water spray column); 6: FT-IR spectrometer; 7: airejector; 8: bypass valve.

Exhaust gas 9 containing PFCs, oxidizing gases, acidic gases and CO isfirst passed through solids separator 1 consisting of a spray columnwhere solids and Si compounds are eliminated. Then, it is passed throughcatalytic treatment apparatus 3 packed with the present γ-alumina 2where H₂, O₂ and/or H₂O are introduced to decompose the PFCs, oxidizinggases and CO into acidic gases and CO₂. At a late stage, it is furtherfreed of the acidic gases in acidic gas treatment apparatus 5 consistingof a spray column and discharged as treated gas 10. Preferably, airejector 7 is provided to control the pressure in these treatmentapparatus and FT-IR spectrometer 6 is incorporated into the system tomanage the treated gas. As for the water used in the spray columns,water 11 is introduced into the spray tower of acidic gas treatmentapparatus 5 and used there, and this used water is directed to the spraycolumn of solids separator 1 via washing water circulating pump 4 andsprayed there and then discharged as wastewater.

EXAMPLES

The following examples further illustrate the present invention without,however, limiting the invention thereto.

Example 1

A γ-alumina having a crystal structure showing diffraction lines havingan intensity of 100 or more at the following five diffraction angles 2θ:33°±1°, 37°±1°, 40°±1°, 46°±1° and 67°±1° available from MizusawaChemical (under trade name “Neobead GB-08”; Na₂O content 0.01 wt % orless) having a particle diameter of 0.8 mm was used. The crystalstructure of Neobead GB-08 was confirmed by using the X-raydiffractometer Rigaku RINT-2000 with CuKα rays as an X-ray source. Thepacked column used was a quartz column having an inner diameter of 25 mmpacked with said γ-alumina at a bed height of 100 mm. This packed columnwas inserted into a ceramic electric tubular oven where the catalyst bedwas heated at 800° C.

A pseudo-exhaust gas consisting of CF₄ diluted in N₂ gas was mixed withH₂ and O₂ used as decomposition-assisting gases in such amounts that theatomic weight of H in H₂ was equal to or more than the atomic weight ofF in CF₄ and O₂ was equimolar to or more than the amount of H₂introduced, and the resulting mixed gas was injected into the packedcolumn at a flow rate of 408 sccm and inlet concentrations of 1 wt %CF₄, 3.0 wt % H₂ and 5.7 wt % O₂ in the mixed gas. In order to evaluatetreatment performance, the treated gas discharged from the outlet of thecolumn was analyzed at appropriate time and gas feeding was stopped todetermine the amount of treated CF₄ from the cumulative amount of feedgas when the CF₄ removal efficiency dropped to 98% or less. The analysisof CF₄ was made by using a gas chromatograph coupled to a massspectrometer.

As a result, the removal efficiency dropped to 98% at 920 min after gasfeeding started and the treated amount was determined from thecumulative amount of CF₄ feed gas at that time to be 77 L/L. The COemission level during then was constantly at or below the permissiblelevel (25 ppm).

Example 2

The same apparatus as in Example 1 was used with the same packing amountof γ-alumina as in Example 1 to perform a treatment as follows.

The temperature was 700° C., the total gas flow rate was 408 sccm andH₂O and O₂ were used as decomposition-assisting gases in which H₂O wasinjected at 0.041 ml/min, i.e., 14 times the flow rate of CF₄ and O₂ wasinjected in an amount equivalent to or more than the necessary number ofmoles for converting C atoms in CF₄ into CO₂. The inlet concentrationsin the resulting mixed gas were 0.88% CF₄ and 3.0% O₂, respectively.

Treatment performance was evaluated in the same manner as in Example 1to show that treatment could be obtained at a removal efficiency of 99%or more up to 74 hours of gas feeding and that the CF₄ removalefficiency dropped to 98% after 94 hours of gas feeding when the treatedamount was 413 L/L. During then, CO was always treated at or below thepermissible level. Examples 1 and 2 were compared to reveal that moreexcellent decomposition performance was shown when H₂O/O₂ (Example 2)ware used as decomposition-assisting gases than H₂/O₂ even if the sameγ-alumina was used.

Comparative Example 1

A control γ-alumina having a crystal structure showing diffraction lineshaving an intensity of 100 or more at the following three diffractionangles 2θ: 37°±1°, 46°±1° and 67°±1° available from Mizusawa Chemical(under trade name “Neobead GB-26”; Na₂O content 0.02 wt %) and dividedand sieved into a particle diameter of 0.8 mm was used as a catalyst.The crystal structure of Neobead GB-26 was confirmed by using the X-raydiffractometer Rigaku RINT-2000 with CuKα rays as an X-ray source.

The same test apparatus as in Example 1 was used with the same packingamount of γ-alumina as in Example 1 to perform a treatment as follows.The temperature was 700° C., the total gas flow rate was 408 sccm andH₂O and O₂ were used as decomposition-assisting gases in which H₂O wasinjected at 0.056 ml/min, i.e., 20 times the flow rate of CF₄ and O₂ wasinjected in an amount equivalent to or more than the necessary number ofmoles for converting C atoms in CF₄ into CO₂. The inlet concentrationsin the resulting mixed gas were 0.86% CF₄ and 3.1% O₂, respectively.

Treatment performance was evaluated in the same manner as in Example 1to show that treatment could be obtained at a removal efficiency of 99%or more up to 33 hours of gas feeding and that the CF₄ removalefficiency dropped to 98% after 50 hours of gas feeding when the treatedamount was 214 L/L. During then, CO was always treated at or below thepermissible level.

Comparative Example 2

A control γ-alumina having a crystal structure showing diffraction lineshaving an intensity of 100 or more at the following three diffractionangles 2θ: 37°±1°, 46°±1° and 67°+1° available from Mizusawa Chemical(under trade name “Neobead GB-45”; Na₂O content 0.01 wt % or less) anddivided and sieved into a particle diameter of 0.8 mm was used as acatalyst. The crystal structure of Neobead GB-45 was confirmed by usingthe X-ray diffractometer Rigaku RINT-2000 with CuKα rays as an X-raysource.

The same test apparatus as in Example 1 was used with the same packingamount of the control γ-alumina as in Example 1 to perform a treatmentas follows. The temperature was 700° C., the total gas flow rate was 408sccm and H₂O and O₂ were used as decomposition-assisting gases in whichH₂O was injected at 0.057 ml/min, i.e., 20 times the flow rate of CF₄and O₂ was injected in an amount equivalent to or more than thenecessary number of moles for converting C atoms in CF₄ into CO₂. Theinlet concentrations in the resulting mixed gas were 0.87% CF₄ and 3.1%O₂, respectively.

Treatment performance was evaluated in the same manner as in Example 1to show that treatment could be obtained at a removal efficiency of 99%or more up to 27 hours of gas feeding and that the CF₄ removalefficiency dropped to 98% after 41 hours of gas feeding when the treatedamount was 177 L/L. During then, CO was always treated at or below thepermissible level.

Comparative Example 3

A control γ-alumina having a crystal structure showing diffraction lineshaving an intensity of 100 or more at the following three diffractionangles 2θ: 37°±1°, 46°±1° and 67°±1° available from Mizusawa Chemical(under trade name “Neobead RN”; Na2O content 0.48 wt %) and divided andsieved into a particle diameter of 0.8 mm was used as a catalyst. Thecrystal structure of Neobead RN was confirmed by using the X-raydiffractometer Rigaku RINT-2000 with CuKα rays as an X-ray source.

The same test apparatus as in Example 1 was used with the same packingamount of the control γ-alumina as in Example 1 to perform a treatmentas follows.

The temperature was 700° C., the total gas flow rate was 408 sccm andH₂O and O₂ were used as decomposition-assisting gases in which H₂O wasinjected at 0.055 ml/min, i.e., 20 times the flow rate of CF₄ and O₂ wasinjected in an amount equivalent to or more than the necessary number ofmoles for converting C atoms in CF₄ into CO₂. The inlet concentrationsin the resulting mixed gas were 0.84% CF₄ and 3.1% O₂, respectively.

Treatment performance was evaluated in the same manner as in Example 1to show that 2950 ppm CF₄ was detected at the outlet after 2 hours ofgas feeding when the removal efficiency dropped to 63%.

Example 2 and Comparative examples 1-3 using the samedecomposition-assisting gases (H₂O/O₂) were compared to reveal that theγ-alumina catalyst having a specific crystal structure according to thepresent invention shows very excellent decomposition performance ascompared with prior γ-aluminas.

Industrial Applicability

According to the present invention, exhaust gases containing noxious andglobal warming fluorine-containing compounds such as PFCs dischargedfrom semiconductor-manufacturing processes can be subjected todecomposition treatment with high decomposition efficiency and highdecomposition treatment performance for a long period. According topreferred embodiments of the present invention, not only PFCs but alsooxidizing gases such as F₂, Cl₂ and Br₂; acidic gases such as HF, HCl,HBr, SiF₄, SiCl₄, SiBr₄ and COF₂; and CO can be efficiently treated.

What is claimed is:
 1. A process for treating an exhaust gas containinga fluorine-containing compound with a catalyst after solids in saidexhaust gas have been separated, said catalyst being a γ-alumina havinga crystal structure showing diffraction lines having an intensity of 100or more at the following five diffraction angles 2θ: 33°±1°, 37°±1°,40°±1°, 46°±1° and 67°±1° measured by an X-ray diffractometer.
 2. Theprocess for treating an exhaust gas containing a fluorine-containingcompound according to claim 1 wherein said γ-alumina is heated at600-900° C. and at least one decomposition-assisting gas selected fromH₂, O₂ and H₂O is further added.
 3. The process for treating an exhaustgas containing a fluorine-containing compound according to claim 2,further comprising the step of eliminating acidic gases from the treatedexhaust gas.
 4. The process for treating an exhaust gas containing afluorine-containing compound according to claim 1, further comprisingthe step of eliminating acidic gases from the treated exhaust gas.
 5. Asystem for treating an exhaust gas containing a fluorine-containingcompound comprising a solids separator for separating solids from saidexhaust gas containing a fluorine-containing compound and a catalytictreatment apparatus for catalytically treating the exhaust gas from saidsolids separator, said catalytic treatment apparatus being packed with acatalyst consisting of a γ-alumina having a crystal structure showingdiffraction lines having an intensity of 100 or more at the followingfive diffraction angles 2θ: 33°±1°, 37°±1°, 40°±1°, 46°±1° and 67°±1°measured by an X-ray diffractometer.
 6. The system for treating anexhaust gas containing a fluorine-containing compound according to claim5, further comprising a means for adding at least onedecomposition-assisting gas selected from H₂, O₂ and H₂O to the exhaustgas from said solids separator.
 7. The system for treating an exhaustgas containing a fluorine-containing compound according to claims 6,further comprising an acidic gas eliminating apparatus for eliminatingacidic gases from the exhaust gas from said catalytic treatmentapparatus.
 8. The system for treating an exhaust gas containing afluorine-containing compound according to claim 6, comprising a meansfor heating the γ-alumina in said catalytic treatment apparatus at600-900° C.
 9. The system for treating an exhaust gas containing afluorine-containing compound according to claims 8, further comprisingan acidic gas eliminating apparatus for eliminating acidic gases fromthe exhaust gas from said catalytic treatment apparatus.
 10. The systemfor treating an exhaust gas containing a fluorine-containing compoundaccording to claim 5, comprising a means for heating the γ-alumina insaid catalytic treatment apparatus at 600-900° C.
 11. The system fortreating an exhaust gas containing a fluorine-containing compoundaccording to claims 10, further comprising an acidic gas eliminatingapparatus for eliminating acidic gases from the exhaust gas from saidcatalytic treatment apparatus.
 12. The system for treating an exhaustgas containing a fluorine-containing compound according to claims 5,further comprising an acidic gas eliminating apparatus for eliminatingacidic gases from the exhaust gas from said catalytic treatmentapparatus.